This invention relates to an evaporative emission control apparatus for a combustion engine and more particularly to a fuel evaporative emission control apparatus having a carburetor fuel shut-off valve.
The California Air Resources Board, CARB, and the United States Environmental Protection Agency, EPA, have been monitoring and regulating the emissions of automobile and non-automobile engines for decades. A source of air born pollution, notably hydrocarbons, has recently been the subject of proposed regulation by CARB. The source is diurnal emissions from lawn and garden equipment such as walk behind lawn mowers, tillers and garden tractors. The vast majority of this equipment is powered by small two and four stroke engines, which use two-way vented gas tanks and either float-bowl or diaphragm carburetors. The hydrocarbon emissions come from the evaporation of gasoline vented to the atmosphere. This equipment is often stored in sheds which are poorly ventilated producing high temperature conditions in excess of one hundred and twenty degrees Fahrenheit (120xc2x0 F.) and/or are often exposed to direct sunlight in the summer.
To reduce evaporation of fuel from this equipment, outward movement of air and fuel vapor from the fuel tank must be prevented. This is already common on hand held equipment powered by two stroke engines, such as string trimmers, blowers hedge trimmers, etc. In the case of handheld equipment, this is done to prevent gasoline from exiting the tank when the equipment is held in an attitude such that the vent is below the level of fuel in the tank and there is a positive pressure in the tank. The positive pressure is common and is caused by a temperature increase in the fuel or vibration of the gas tank. In addition to sealed gas tanks on handheld equipment, the vast majority of this equipment employs two stroke engines with sealed crankcases and diaphragm type carburetors. Interestingly, this handheld equipment produces much less evaporative emissions than the non-handheld equipment utilizing float-bowl carburetors, and CARB is proposing only a less permeable gas tank material for this equipment. Unfortunately, diaphragm carburetors are not practical for all applications. They have limited fuel metering and vapor vent capability which can lead to engine instability and vapor lock conditions.
Float-bowl carburetors on the other hand have higher fuel metering capability and are commonly used on engines powering non-handheld lawn and garden equipment. The float-bowl carburetor is a relatively simple mechanical device and is known for high evaporative emissions. The float-bowl carburetors used on the smaller engines such as walk behind lawn mowers are the simplest of all. They do not utilize fuel pumps, mounting the gas tank above the carburetor and relying on gravity to feed the fuel. These carburetors do not even have an idle system and the engine operates at a relatively constant speed from no load to full load operation. Cost is a major driver in this market as there are many competitors chasing this multi-million engine per year market. These engines also employ the simplest of ignition systems with electric power generated only for the ignition to fire the spark plug. Therefore, a solution to reduce evaporative emissions should be simple, mechanical and cost effective to be viable for this market.
The diaphragm type carburetors in use today on most hand held equipment, by their design, do not allow the passage of fuel from the gas tank into the metering chamber of the carburetor unless the engine is running and there is sufficient vacuum generated in the carburetor metering chamber to depress the metering diaphragm that opens the spring bias closed inlet valve. The inlet valve is spring loaded closed and it is common that it will remain closed against inlet pressures exceeding twenty psi. By contrast, the typical float-bowl carburetor has an inlet valve which is normally biased open unless the float bowl is completely full of fuel thus closing the valve. The volume of fuel contained in a typical float bowl is several time greater than that of the metering chamber of a diaphragm type carburetor. The gasoline commonly used today evaporates over a wide temperature range starting at around ninety degrees Fahrenheit (90xc2x0 F.) with approximately thirty percent (by volume) gone by one hundred sixty degrees Fahrenheit (160xc2x0 F.) and ninety plus percent (by volume) gone at three hundred fifty degrees Fahrenheit (350xc2x0 F.).
When a piece of lawn and garden equipment is shut down after a sufficient amount of running time that the engine is at normal hot operating temperatures, the first thing that happens (over thirty minutes) is that heat is transferred from the cylinder head of the engine thru the intake manifold to the carburetor. The carburetor, which may have been at a sub-ambient temperature while running due to the cooling effect of the vaporization of the gasoline in the venturi heats up. The fuel that is in the metering chamber of a diaphragm carburetor or the float bowl of a float carburetor evaporates by a volume percent dependent on the highest temperature reached, and from this point the temperatures of the entire piece of equipment cools to ambient. Now the equipment is placed in a lawn shed with limited ventilation. Assume the temperature fluctuates over a twenty-four hour period from a daytime high of one hundred twenty degrees Fahrenheit to a nighttime low of sixty degrees Fahrenheit. First consider the equipment with the diaphragm type carburetor. Assume that the carburetor reached a temperature in excess of one hundred twenty degrees Fahrenheit after the equipment shutdown. A percentage of fuel in the metering chamber would evaporate and go into the atmosphere (say twenty percent (by volume)). Since the engine is not started during storage, the twenty percent of the fuel lost from the metering chamber after shut down is not replaced from the tank. During the diurnal temperature swings, since the daytime temperature does not exceed the initial temperature, no further fuel is evaporated, even if the daytime temperature does exceed the previous high, the evaporation loss is only the percent difference between the previous and the new high.
Now consider a typical float-bowl carburetor, with the same temperatures, during the initial heat and soak back from the engine to the carburetor, twenty percent of the fuel in the bowl evaporates into the atmosphere. This is a greater amount of fuel loss than the diaphragm carburetor due to the fact that the float bowl held more fuel than the diaphragm chamber. To further aggravate matters, when the fuel evaporates the float drops allowing fresh fuel from the sealed gas tank to replace the evaporated fuel. This will continue with each subsequent temperature rise until the entire float bowl is filled with fuel that does not evaporate at the highest temperature reached. Therefore to minimize the loss of fuel due to evaporation from a float-bowl carburetor, refilling of the float bowl with fuel must be prohibited and preferably, the volume of fuel in the bowl should be minimized. A manual shut-off valve at the bottom of the fuel tank or at the fuel inlet of the carburetor is common, however not sufficient because there is no assurance that the ordinary user of the lawn and garden equipment will close the valve on shutdown.
This invention provides a fuel evaporative emission control apparatus which prevents the diurnal evaporation and exposure to the atmosphere of fuel from a remote fuel tank via a fuel bowl or fuel chamber of a float-type carburetor. A fuel shut-off valve is mounted on the float-type carburetor and preferably a bowl vent shut-off valve is installed in the bowl vent passage of the carburetor with both valves biased to their shut-off position when the engine is not being cranked or running. Only upon engine operation or user intervention can the shut-off valves move from the biased off position to an open position. In this way, reliance upon the user is not required to reduce evaporative emissions.
The fuel shut-off valve is part of a carburetor body and has a recess between the carburetor body and a lid of the shut-off valve. A valve head is received in the recess and obstructs communication between an inlet aperture and outlet aperture communicating with the recess. A fuel-in passage communicates between the inlet aperture and an external carburetor fuel inlet which leads to the external fuel tank. A fuel-out passage communicates between the outlet aperture and the fuel chamber.
Preferably, the bowl vent shut-off valve has a ball disposed in a counterbore carried by the carburetor body. The ball, via gravity, seats against a seat insert press fitted into the counterbore, thereby preventing fuel evaporation and escape through the vent passage from the float bowl.
A float is disposed within the fuel chamber and operates a needle valve to close the fuel-out passage when the fuel chamber is full, and to open the fuel-out passage when the fuel level within the chamber is low. The shut-off valve operates to isolate the fuel-in passage from the fuel-out passage regardless of the needle float position when the engine is not running. In this way, evaporation and escape to the atmosphere of fuel contained in the remote fuel tank of a non-running engine is prohibited.
Objects, features and advantages of this invention include providing an evaporative emission control apparatus which limits evaporative emissions, does not require operator intervention to activate it, has an extremely compact construction and arrangement, is of relatively simple design, extremely low cost when mass produced and is rugged, durable, reliable, requires little to no maintenance and in service has a long useful life.